The inspection of newly-manufactured hardware for aircraft and the like can generally be handled by an inspection facility located close to a manufacturing site having test equipment designed for production compatibility.
In the field, however, in-service inspection is most often performed by portable test equipment for hand scanning. This can be a slow operation when large surfaces are involved. Further, in the case of complex structures, such as bonded aluminum, honeycomb parts with chem-milled skins, stiffeners, access ports and the like, either a hand-scanning technique or some sort of portable small scale scan-record system is required. For large structures with uniform instrument response over large areas, a test technique is required where a wide path can be swept. This currently, however, often requires one or more sensing devices.
Recent studies have found evidence of the limitations of “hand scanning” and the need for some kind of tool to assure a complete coverage of an area. During maintenance of aircraft, there are times when “area” nondestructive inspections are required. There are several situations where this can arise as part of a scheduled maintenance procedure, such as the rudder and elevator ultrasonic inspections specified at certain intervals for different aircraft types.
As a result of damage found on one or more aircraft in a fleet, certain aircraft are selected to undergo surveillance inspections to find damage suspected to exist in locations not exactly known. An example of suspected damage is the disbanding that may arise anywhere in the entire surface of a wing fixed trailing edge panel.
These damage inspections cover entire structure areas having no apparent damage indications, as opposed to the more straightforward practice of scanning localized areas associated with visible impact or lightning strike. It is therefore necessary to “index” the inspection transducer or probe carefully to ensure that the areas are completely covered, leaving no gaps that could contain the small flaws sought. Since the transducer or probe leaves no visible evidence of where it has been, this is no easy task.
The Mobile Automated Scanner (MAUS), or any similar portable C-scan device, is one such tool that assures complete scan coverage. However, airlines have been reluctant to use portable C-scan equipment due to the high cost ($50K to $100K) and the cumbersome nature of the equipment. Repeatedly, airlines have shown their preference for a cheaper, more portable way of accomplishing the same inspection. Smaller carriers, in particular, with only a few airplanes in their fleet, have been reluctant to invest capital in portable C-scans that spend much of their lives collecting dust on a shelf.
An alternative method for assuring complete scan coverage is to use a straightedge to guide the probe across the part. This enables the inspector to scan parallel to underlying stiffeners (a critical technique for interpreting scan signals) and to scan in straight, parallel lines without wandering. It is, however, difficult to hold a straightedge in place while scanning a probe and simultaneously monitoring a screen display. Furthermore, it is difficult to accurately index the straightedge and keep it parallel to the original path after each scan path. And finally, it is nearly impossible to fix the straightedge to the part with double-backed tape or other common adhesive due to the presence of ultrasonic couplant generally found on part surfaces.
There is, therefore, a need for a rapid scanning method for testing aircraft and the like which, at the same time, will accurately and precisely locate any defects or damage in the aircraft structure. Further, there is a need for improved methods and apparatus for scanning aircraft and, in particular, for a guide for a low-cost transportable scanning system that easily attaches to aircraft during a scanning operation.